Update 3: A “special Pokémon” was briefly, verbally teased during the conference. We can confirm this is a “new Pokémon” that’s coming to the Let’s Go! games.
Update 2: Players will be able to bring all 151 Kanto-region Pokémon from Pokémon Go to Pokémon Let’s Go! Pikachu and Eevee, as well as the Alolan forms of those Pokémon such as Alolan Exeggutor (which has just been surprise-released to Pokémon Go, too).
Update 1 (correction): The Let’s Go! core RPG games will release November 16th, 2018. The mobile-switch hybrid game Pokémon Quest is out now, while an additional, “core RPG” Pokémon game will release “in the second half of 2019”.
Source: Eurogamer Pokémon Let's Go! Pikachu and Eevee confirmed for Nintendo Switch, featuring Pokémon Go-style catching